This invention relates to a high-sensitivity differential refractometer of improved measuring precision for use in liquid chromatography.
Every substance has its own index of refraction. The high-sensitivity differential refractometer is an instrument for measuring the index of refraction of a solution subjected to measurement by means of a junction-prism type cell containing in one end section thereof a solvent serving as the reference specimen and in the other end section thereof a test specimen on said solution, with the measurement being made by utilizing the change in the index of refraction of light which is caused when a light is passed through said cell.
The high-sensitivity differential refractometer (hereinafter referred to as "refractometer") permits even a very slight change of concentration in any solution to be measured continuously and with high accuracy in terms of the difference in refractive index between a test specimen of said solution and a solvent serving as the reference specimen. Owing to the high sensitivity of the measurement, this instrument has found extensive adoption as a detector in the liquid chromatography.
Generally, the refractometer has as its principal components, a cell adapted to pass the solvent and the test specimen into the interior of the housing so as to cause a refraction of light corresponding to the difference between the indices of refraction of the two liquids, a light-source lens adapted to pass the light through the cell, a mirror or some other similar article serving to reflect the incident light, an optical base for holding said mirror in position and an element for detecting the reflected light. The index of refraction of a solution is very sensitively affected by the temperature of said solution. In the detection of a very minute change in the index of refraction of the order of 1 .times. 10.sup.-7, for example, a change of temperature as slight as 0.001.degree. C. can cause a large enough drift of the base line to degrade the accuracy of measurement. This means that during actual measurement the temperature change in the cell interior and the difference of temperature between the liquid held inside the cell and the solvent must be minimized as much as possible.
Given below are the three major causes for temperature change or ununiform temperature distribution in the cell interior.
A. Local heating by light-source lamp. PA1 b. Thermal effect exerted externally. PA1 c. Temperature difference between the test specimen passed into the cell interior and the solvent and consequent change of temperature.
The measure conventionally adopted to cope with the cause "a"is to provide the lamp with a cooling device or to select a lamp of the kind designed to suppress heavy generation of heat. The former measure has the disadvantage that the addition of said extra device complicates the refractometer as a whole and adds to the price thereof. The latter measure entails a degradation in the accuracy of the refractometer.
The conventional measure adopted to cope with the cause of "b" is to wrap the optical base in a heat insulating material.
The measure taken to cope with the cause of "c" is to keep the temperature of the specimen as close to the temperature of the cell as permissible by either causing the lead-in pipe forming a passage between the exterior and the interior of the cell to be buried in a metallic block or powder having a great heat capacity or passing the pipe through a heat-exchange unit. This pipe serves the purpose of forwarding to the cell the separated liquid which is discharged from the liquid chromatograph. To prevent the pipe from permitting undesirable mixing of solution, the length (inner volume) of the pipe must be reduced to the tolerable minimum. The pipe to the cell is generally made of a highly anticorrosive stainless steel pipe having an inside diameter of not more than 0.5 mm. The flow rate of the liquid within this pipe is in the range of from 4 to 10 cm per second. And the pipe has a limited length. With the pipe of such a description, it is difficult to provide ample exchange of heat. When the refractometer is operated to make the measurement, the optical axis or the position of the reflecting mirror must be adjusted so that the light projected from the light source, passed through the array of lenses and reflected by the reflecting mirror will impinge correctly upon the desired position on the detection unit.
In the conventional refractometer, the reflecting mirror is attached in position in the manner described below.